Radio communication systems generally provide two-way voice and data communication between remote locations. Examples of such systems are cellular and personal communication system (PCS) radio systems, trunked radio systems, dispatch radio networks, and global mobile personal communication systems (GMPCS) such as satellite-based systems. Communication in these systems is conducted according to a pre-defined standard. Mobile stations, also known as handsets, portables or radiotelephones, conform to the system standard to communicate with one or more fixed base stations.
It is desirable to obtain and communicate physical locations of mobile stations within a system, such as radiotelephone handsets within a cellular system. In addition, the United States Federal Communications Commission (FCC) has required that cellular handsets must be geographically locatable by the year 2001. This capability is desirable for emergency systems such as Enhanced 911 (E911). The FCC requires stringent accuracy and availability performance objectives and demands that cellular handsets be locatable within 100 meters 67% of the time for network based solutions and within 50 meters 67% of the time for handset based solutions.
Further, with location information available for mobile stations, position-dependent services and messaging including advertising can be tailored to the handset user responsive to the location of the handset.
Current generations of radio communication have only limited mobile station location determination capability. In one technique, the position of the mobile station is determined by monitoring mobile station transmissions at several base stations. From time of arrival measurements, the mobile station's position can be calculated. However, the precision of this technique is limited and, at times, may be insufficient to meet FCC requirements.
In another technique, each mobile station is equipped with a receiver suitable for use with a global satellite navigation system such as the Global Positioning System (GPS). The construction and operation of receivers suitable for use with GPS are described in U.S. Pat. Nos. 5,175,557 and 5,148,452, both of which are assigned to the assignee of the present invention. The GPS receiver detects transmissions from a constellation of GPS satellites orbiting the Earth. Using data and timing from the transmissions, the GPS receiver calculates the positions of the satellites and from those positions, its own position. A GPS satellite in orbit moves at about 4,000 meters per second. The satellite has location data defined by a parameter X(t) and velocity data defined by a parameter V(t). The parameters X(t) and V(t) are three-dimensional position and velocity vectors for this satellite and are referenced to an earth-centered, earth-fixed Cartesian coordinate system. The GPS system includes 24 satellites, several of which may be in view of the mobile station at any one time. Each satellite broadcasts data according to pre-defined standard formats and timings.
Traditionally, the satellite coordinates and velocity have been computed inside the GPS receiver. The receiver obtains satellite ephemeris and clock correction data by demodulating the satellite broadcast message stream. The satellite transmission contains 576 bits of data transmitted at 50 bits per second (bps). The constants contained in the ephemeris data coincide with Kepler orbit constants requiring many mathematical operations to turn the data into position and velocity data. In one implementation, this conversion requires 90 multiplies, 58 adds and 21 transcendental function calls (sin, cos, tan) in order to translate the ephemeris into a satellite position and velocity vector at a single point, for one satellite. Most of the computations require double precision, floating point processing. A receiver must perform this computation every second for every satellite, for up to twelve satellites.
Thus, the computational load for performing the traditional calculation is significant. The handset must include a high-level processor capable of the necessary calculations. Such processors are relatively expensive and consume large amounts of power. As a portable device for consumer use, a mobile station is preferably inexpensive and operates at very low power. These design goals are inconsistent with the high computational load required for GPS processing.
Further, the slow data rate from the GPS satellites is a limitation. GPS acquisition at a GPS receiver may take many seconds or several minutes, during which time the receiver circuit and processor of the mobile station must be continuously energized. Preferably, to maintain battery life in portable receivers and transceivers such as mobile cellular handsets, circuits are de-energized as much as possible. The long GPS acquisition time can rapidly deplete the battery of a mobile station. In any situation and particularly in emergency situations, the long GPS acquisition time is inconvenient.
One system proposal, termed Assisted GPS, includes receiving the ephemeris and clock correction data at a base station of the radio communication system and transmitting this data over a conventional communication link to the mobile station. The base station receives the 50 bps transmission from a GPS satellite and acts as a repeater, gathering the data from the GPS satellite and re-transmitting it at a higher data rate to a mobile station. The ephemeris and clock correction data are received at the mobile station and used for calculation of satellite position. From satellite position, mobile station position can in turn be determined. While this system has some advantages such as a greater data rate used in the base-to-mobile communication link (typically as high as 9600 bps) to allow the mobile station's receiver circuit to be turned off a greater amount of time, the high computational load associated with the raw ephemeris data remains.
Another proposed solution stores a GPS almanac at the mobile station. The almanac data is a truncated, reduced precision subset of the ephemeris data. A base station computes location and clock correction information for the almanac and transmits this correction over the communication link to the mobile station. The mobile station determines that it has the proper correction data for its almanac and, if so, computes satellite location and clock data using the almanac.
This system reduces slightly the computational load required of the mobile station. However, the mobile station receiver must still remain energized during transmission of its almanac data and subsequently during all possible transmission times to receive correction data for its almanac. Also, the almanac data must be stored at the mobile station, which can increase the size and cost of the mobile station.
Yet another proposed solution is a network-centric approach that places the responsibility of determining when a mobile should be updated in the network. This places a harsh constraint on the end-application, requiring the most stringently applied database update requirements to be applied to all mobiles, as there is no provision by which the mobile can inform the network how to prioritize the database update scheme.
There are three major drawbacks to this proposed solution. First, it has not been proven that the dynamic range of the incremental update fields will be sufficient to cover all possible values of the parameters. Given the complex nature of the incremental update algorithms, using several weeks of ephemeris data may not sufficiently cover all possibilities, such as satellite station-keeping maneuvers or satellite orbit changes that are periodically performed by the GPS ground control segment. In the past, several GPS satellites were repositioned to higher orbits for weeks at a time in order to place these particular satellites into a different part of the orbit plane. The satellites were active during the entire re-orbit phase, except for short periods before and after the acceleration and braking events.
Second, a new parameter, Issue of Data Assistance (IODA), must be created and used for every sampling of the visible satellites in every fifteen minutes, and complicated intra- and inter-tables as well as related data structure and algorithms must be created, maintained, communicated at the Serving mobile location Center (SMLC), along with the IODA. In particular, the inter-table maintenance and inter-SMLC communication is difficult and is often viewed as not necessary.
And third, while once the mobile has obtained a raw ephemeris set for a visible satellite, there is not a need to update that ephemeris set in most, if not all, cases, the prior art proposed solution suggests frequently updating the ephemeris data. This occupies considerable portions of the point-to-point transmission channel as shown in Table 1 below:
TABLE 1 Total Bits Delivered for the Prior Art to the MS per Visible Satellite % SV's with % SV's with % SV's with % SV's with % SV's with Raw EPH 2 hr Increm 4 hr Increm 6 hr Increm 8 hr Increm Total Raw Equiv Latitude Update EPH Update EPH Update EPH Update EPH Update Updates per SV 60 Deg 100% 85% 50% None None 1.68 .times. Raw EPH Bits 30 Deg 100% 90% 70% 30% None 1.95 .times. Raw EPH Bits 15 Deg 100% 95% 80% 30% 15% 2.10 .times. Raw EPH Bits Equator 100% 97% 85% 65% 35% 2.41 .times. Raw EPH Bits
Accordingly, there is a need for an improved method and apparatus to support location determination in a radio communication system.